The invention relates to a high-impedance and wide-band voltage probe for the measurement of balanced or unbalanced interference voltages or interference voltages occurring between each terminal and ground of electrical or electronic systems connected to single-phase a.c. or d.c. sources.
The need for separate measurement of the unbalanced and balanced interference voltages of electronic apparatus and/or systems had already been recognized since the development of interference suppression techniques.
The solutions proposed by "Comite International Special des Perturbations Radioelectriques" (CISPR) are intended for the measurement of such voltages using mains simulation under precisely defined conditions (impedance of the mains) and are confined to apparatus with a maximum current consumption of 16 Amps. A high-impedance interference voltage voltmeter was initially used but in the course of time mains simulations were adapted to low-impedance measuring apparatus.
Measurement of interference voltages on apparatus and systems with a greater power and under operating conditions led to increased difficulties and the CISPR limited itself to measurement of interference voltages relative to ground (earth). A voltage probe with an impedance of 1500 Ohms between the measuring point and ground is universally employed at the present time. The limit values of the interference voltages are also referred to ground.
In connection with interference suppression work on trolleybuses, the relevant organizations of the Swiss Postal Telephone and Telegraph Administration developed a measuring method for defining the unbalanced interference voltages by the use of the resonant circuits. The disadvantage of this method is the necessity for tuning the circuits and the accompanying possible lack of symmetry.
It is frequently the case, for example, in interference suppression work, that measurement of the unbalanced or balanced interference voltages is desirable in systems under operating conditions and in some circumstances even necessary; this can apply to the optimum design of interference suppression means, taking into account protection against electric shock. Such measurement is possible only to a limited extent when using the above-mentioned mains simulations; on the one hand, the mains current is limited to 16 Amps and on the other hand it is not possible to measure objects which are driven through current-adjusting means.